Is it illegal?

I know a person that textes someone who is currently underage. They're not adult, but they aren't underage. The another person isn't a little kid, just still underage (14 years old actually.) They mostly communicate normally, but sometimes the conversation turns to kinda inappropriate topic. There aren't any nudes or pornographic images, just mentioning sex in general, but not that they want it together, just someone else, who's adult might have it with another adult. No describing, just mentioning.
Is it actually illegal, or just inappropriate?